The present invention relates to plankton removal from fish pens, more particularly to an improved system for displacing plankton from fish pens.
Fish rearing in pen, for example, in open net pens that usually but not exclusively are enclosed by open mesh, are exposed to the conditions prevailing in the water surrounding the pen.
Several undesirable conditions can occur in this water including, but not limited to:
1. Presence of toxic or parasitic plankton (usually algae or sealice).
2. Undesirable water temperature (usually too high).
3. Undesirable oxygen concentrations in the water (usually too low).
In natural bodies of water stratification of the water column can occur. This can lead to a condition where an undesirable condition is present in the upper part of the column but not below a certain depth. Examples of this condition include algal blooms where the significant portion of the bloom is in the euphotic or illuminated part of the column and thermal stratification wherein warmer water overlies colder water with a distinct thermocline between them. Often these phenomena are associated and sometimes linked.
Operators in some areas often use multiple air-lift pumps to provide algae-free water during algal blooms. These pumps are small, so many are required and do not discharge in a radial fashion. In addition, the flow rate achievable is low so that they often fail to provide adequate mitigation. Typically, these pumps would be arranged around the perimeter of a tarped net-pen system discharging in over the tarp.
Applicant is aware of one operator in Chile who has tried a simple vertical pipe with a pump inside it and discharges from the pipe in a substantially vertical direction. The reports have had reports that this strategy was not very successful. We have no data on flow rates so and therefore cannot estimate current or downfield dispersion.
One example of aeration and/or circulating devices is shown in U.S. Pat. No. 5,564,828 issued Oct. 15, 1996 to Haegeman which shows a system that takes water from in the pen moves it vertically and then disperses it with a tangential component to disperse and mix the water in the tank or pen. U.S. Pat. No. 4,350,648 issued Sep. 21, 1982 to Watkins and U.S. Pat. No. 5,110,510 issued May 5, 1992 to Norcross each describe recirculation systems wherein the output is diverted radially and adjustment are provided to control the output and how the water is recirculated, i.e. in Watkins a diffuser cone is connected to a diffuser hood that move together between a mixing position wherein the cone blocks the flow of water through float on which the system is mounted and an aerating position wherein the diffuser hood is positioned against the float and the cone is spaced above the float to divert the water.
U.S. Pat. No. 4,798,168 issued Jan. 17, 1989 to Vadseth et al. describes a system wherein water is pumped (air lifted) from below and injected tangentially into a peripherally closed tank to expel water in the tank through an overflow outlet. This patent does refer to drawing water at a suitable temperature form below, but they are discussing operation in cold temperatures where the water below is at a higher temperature than the water in the tank and they use the higher temperature water from below to raise the temperature in the tank.
It is the main object of the invention to provide a system (method) for removing plankton from a fish pen.
Broadly the present invention relates to a method of removing plankton from a fish pen comprising pumping a cleaning water from a depth sufficiently low to provide a source of cleaning water having a density significantly higher than the water density of water in said pen, said pumping delivering higher density cleaning water at a velocity sufficient to distribute said higher density water across at least a major portion of the area of said pen while permitting at least a portion of said higher density water to sink toward a bottom of said pen at a rate to,carry plankton with it and dispensing said plankton carried to said bottom by said cleaning water from said pen.
Preferably said pumping dispenses said cleaning water substantially radially of an outlet without formation of an abrupt downward plume of water at said outlet.
Preferably said pumping dispenses said cleaning water substantially radially of an outlet to form a surface plume that extends over a major portion of the area of the surface of said pen. Preferably said pumping dispenses said cleaning water at a velocity having a component radially of said outlet of between 1 and 3 meters/second.
Preferably said pumping dispenses said cleaning water at a flow rate of between 1 and 2 cubic meters/second.
Preferably said cleaning water in said pen has a downward velocity obtained substantially solely based on turbulence and the difference in density between said cleaning water and said water in said pen.